Multi-stage centrifugal pumps are known, with which several pump stages, in each case consisting of a pump impeller and a spiral housing which surrounds this, are arranged between a head part and a foot part, wherein the impellers are arranged on a common shaft. Thereby, the head part and foot part amid the inclusion of the pump stages are connected to one another via outer-lying tie rods in the form of screws.
Such a centrifugal pump typically comprises four screws, which run on the outside along the pump stages. Thereby, embodiments are known, with which the spiral housing in the region of the pump stages form the outer casing or such, with which a fluid return is effected within the housing, typically to the foot part, and which comprise an outer casing which forms an annular channel between the outer sides of the spiral housing and the outer casing, via which annular channel the delivery fluid flows from the head part to the foot part or, as the case may be, also vice versa.
Common to both embodiments is the fact that the screws, with which the head part and foot part are clamped amid the inclusion of the pump stages or, as the case may be, of the outer casing surrounding these, bear in the region of the head part and foot part, but have a certain distance in the region of the pump stages. The latter fact leads to the fact that the temperature of the screws can differ significantly from that of the delivery fluid and thus also that of the spiral housing or of the outer casing, which leads to thermal stresses within the centrifugal pump. Such thermal stresses can also lead to premature wear or failure of the pump.
A further disadvantage of this construction type is the fact that not only the shaft and, as the case may be, the outer casing must be provided in different lengths depending on the number of pump stages, but also that the screws connecting the head part and foot part must be provided in different lengths, in order depending on the number of pump stages, to connect the head part and foot part amid the inclusion of the pump stages.